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Java Json字符串解析
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这里使用fastjson来进行解析，fastjson里有两个重要的对象JSONObject和JSONArray。看下面的json字符串，meta、data属于JSONObject，data子集因为有[ ]所以子集属于JSONArray。
1 {
2 "meta": {
3 "page": 1,
4 "limit": 10000,
5 "hasnext": false,
6 "count": 1
7 },
8 "data": [
9 {
10 "recv_cnt": 110,
11 "proto_name": "MQTT",
12 "clientid": "mqttx_7c2fecaa",
13 "connected_at": "2020-11-29 12:28:18",
14 "mailbox_len": 0,
15 "send_pkt": 111,
16 "recv_oct": 294,
17 "max_mqueue": 1000,
18 "send_msg": 2,
19 "connected": true,
20 "mountpoint": "undefined",
21 "max_subscriptions": 0,
22 "heap_size": 610,
23 "subscriptions_cnt": 1,
24 "zone": "external",
25 "max_awaiting_rel": 100,
26 "mqueue_dropped": 0,
27 "is_bridge": false,
28 "expiry_interval": 0,
29 "recv_pkt": 3,
30 "node": "emqx@127.0.0.1",
31 "mqueue_len": 0,
32 "ip_address": "192.168.3.3",
33 "proto_ver": 4,
34 "inflight": 0,
35 "keepalive": 60,
36 "reductions": 87838,
37 "created_at": "2020-11-29 12:28:18",
38 "send_cnt": 111,
39 "awaiting_rel": 0,
40 "username": "undefined",
41 "clean_start": true,
42 "send_oct": 276,
43 "port": 53482,
44 "recv_msg": 1,
45 "max_inflight": 32
46 }
47 ],
48 "code": 0
49 }
1.获取code字段值。
String jsonString = "{"meta":{"page":1,"limit":10000,"hasnext":false,"count":1},"data":[{"recv_cnt":110,"proto_name":"MQTT","clientid":"mqttx_7c2fecaa","connected_at":"2020-11-29 12:28:18","mailbox_len":0,"send_pkt":111,"recv_oct":294,"max_mqueue":1000,"send_msg":2,"connected":true,"mountpoint":"undefined","max_subscriptions":0,"heap_size":610,"subscriptions_cnt":1,"zone":"external","max_awaiting_rel":100,"mqueue_dropped":0,"is_bridge":false,"expiry_interval":0,"recv_pkt":3,"node":"emqx@127.0.0.1","mqueue_len":0,"ip_address":"192.168.3.3","proto_ver":4,"inflight":0,"keepalive":60,"reductions":87838,"created_at":"2020-11-29 12:28:18","send_cnt":111,"awaiting_rel":0,"username":"undefined","clean_start":true,"send_oct":276,"port":53482,"recv_msg":1,"max_inflight":32}],"code":0}"；
JSONObject outJson = JSONObject.parseObject(jsonString);
System.out.println(outJson.getIntValue("code"));
2.获取meta下的“limit“字段值。
String jsonString = "{"meta":{"page":1,"limit":10000,"hasnext":false,"count":1},"data":[{"recv_cnt":110,"proto_name":"MQTT","clientid":"mqttx_7c2fecaa","connected_at":"2020-11-29 12:28:18","mailbox_len":0,"send_pkt":111,"recv_oct":294,"max_mqueue":1000,"send_msg":2,"connected":true,"mountpoint":"undefined","max_subscriptions":0,"heap_size":610,"subscriptions_cnt":1,"zone":"external","max_awaiting_rel":100,"mqueue_dropped":0,"is_bridge":false,"expiry_interval":0,"recv_pkt":3,"node":"emqx@127.0.0.1","mqueue_len":0,"ip_address":"192.168.3.3","proto_ver":4,"inflight":0,"keepalive":60,"reductions":87838,"created_at":"2020-11-29 12:28:18","send_cnt":111,"awaiting_rel":0,"username":"undefined","clean_start":true,"send_oct":276,"port":53482,"recv_msg":1,"max_inflight":32}],"code":0}"；
JSONObject outJson = JSONObject.parseObject(jsonString); //先将原始json字符串转换成JSONObject
String metaString = outJson.getString("meta"); //从JSONObject中获取meta字符串
JSONObject metaObj = JSONObject.parseObject(metaString); //再将meta字符串转换成JSONObject
System.out.println(metaObj.getIntValue("limit")); //获取limit字段值
3.获取data下的clientid字段值。
String jsonString = "{"meta":{"page":1,"limit":10000,"hasnext":false,"count":1},"data":[{"recv_cnt":110,"proto_name":"MQTT","clientid":"mqttx_7c2fecaa","connected_at":"2020-11-29 12:28:18","mailbox_len":0,"send_pkt":111,"recv_oct":294,"max_mqueue":1000,"send_msg":2,"connected":true,"mountpoint":"undefined","max_subscriptions":0,"heap_size":610,"subscriptions_cnt":1,"zone":"external","max_awaiting_rel":100,"mqueue_dropped":0,"is_bridge":false,"expiry_interval":0,"recv_pkt":3,"node":"emqx@127.0.0.1","mqueue_len":0,"ip_address":"192.168.3.3","proto_ver":4,"inflight":0,"keepalive":60,"reductions":87838,"created_at":"2020-11-29 12:28:18","send_cnt":111,"awaiting_rel":0,"username":"undefined","clean_start":true,"send_oct":276,"port":53482,"recv_msg":1,"max_inflight":32}],"code":0}"；
JSONObject outJson = JSONObject.parseObject(jsonString);
String dataString = outJson.getString("data");
JSONArray dataArray = JSONObject.parseArray(dataString);
for (int i = 0; i < dataArray.size(); i++) {
String clientid = dataArray.getJSONObject(i).getString("clientid");
System.out.println(clientid);
}
总结：
1.如果字段是单值，那么原始json字符串转换为JSONObject后直接获取；
2.如果字段是对象，那么需要先获取字段的String再转换为JSONObject，如此循环直到获取到你想要的字段；
3.如果字段是数组，那么需要先将其转换为JSONArray，然后再套用第1和2点。
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